


Venom Cures

by Askellie



Category: Undertale (Video Game)
Genre: Background Character Death, Horror, Implied/Referenced Torture, M/M, Manipulation, Master/Servant, Mildly Dubious Consent, Mind Control, Mind Manipulation, Nightmare is terrible, Power Imbalance, Socket fucking
Language: English
Status: Completed
Published: 2020-10-10
Updated: 2020-10-10
Packaged: 2021-03-08 03:06:57
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,146
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26928610
Author URL: https://archiveofourown.org/users/Askellie/pseuds/Askellie
Summary: Killer can lie and play pretend, but there's no where else he belongs except at Nightmare's side.
Relationships: Kight - Relationship, NightKiller, Nightmare (Dreamtale) / Killer, Sans/Sans (Undertale)
Comments: 6
Kudos: 141





	Venom Cures

**Author's Note:**

> A little gift for 0neType after a [conversation](https://twitter.com/Askellie_ut/status/1310029500428644353) we had on Twitter about how socket-fucking really needs to become more of a kink staple for people who write for the Undertale Multiverse AUs. It was such a big thing in the Fontcest niche, I'm shocked it hasn't caught on more!
> 
> This idea is also based on the awesome socket-fucking mind control headcanon by Shadow of Quill.

Killer is heading to the basement again.

Nightmare pauses, eyes resting sightlessly on the page of the book he’s reading. It’s a rare quiet day in the Castle. No pressing business for him to take care of, no missions for his subordinates. It’s a rare, generous moment of downtime and Killer must be hoping his absence won’t be noticed. No doubt he’s trying to sneak out to Othertale to visit Color. His soul must be teetering towards stability again, if he’s feeling such a sentimental inclination.

Nightmare knows just how to fix that.

Setting his book aside, he lets his body melt into the darkness and reform in the corridor just beyond the basement landing. The thick stone walls are rough-hewn and aged. The castle’s foundation was built by shadow servants, long ago, but despite the inelegant construction there’s more to the space than its empty, maze-like exterior. Inside the walls are small, cramped pockets of space - a honeycomb of tiny prisons. Intermittently he’ll kidnap souls from other AUs and leave them trapped inside the tiny, inescapable cells to die a long, slow, miserable death of starvation and madness. It keeps the base of his castle steeped in a constant outpouring of negativity, a thick miasma invisible to the eye.

He’s never told anyone about these prisoners or their purpose. Killer probably doesn’t understand why he comes to the basement whenever he wants to leave, perhaps only thinking it to be advantageously empty and private. Likely he doesn’t even comprehend the unconscious attraction of all that negativity resonating with the power that’s slowly seeped into Killer’s bones over their years of partnership. His LV has kept his mind from breaking under the onslaught of Nightmare’s aura, but it hasn’t stopped the guardian’s power from affecting the innate qualities of his magic, turning his ability to shortcut into something that can slip between the fabric of worlds as long as there’s enough misery to draw on.

How Killer thinks he can go unnoticed, sneaking out of the castle using Nightmare’s own power is uncertain. Does he care so much about Color that he chooses to risk it?

Or perhaps he simply doesn’t mind getting caught?

He spots Killer ahead of him, heading deeper into the basement. There’s no light except for the prominent glow of Killer’s soul casting an ominous red halo around him, but Nightmare can see perfectly. Despite his nonchalant gait, there’s an alert tension to his spine and shoulders. He’s primed for trouble, but unfortunately for him, Nightmare’s approach is completely silent, masked by the shadows.

“Did you get lost, Killer?”

Killer doesn’t startle. He’s too good for that. But there’s a slight thread of caution in the way he turns around, moving slowly like he knows he’s prey trying not to draw the attention of a predator. 

There’s no hiding the state of his soul. It hovers over Killer’s sternum, its shape steady and color strong. It’s still mostly circular, but there’s a hint of elongation near the top, the beginning of the point like the original shape of his soul.

Killer’s typical smile stretches extra wide, coyly ingratiating. “Hey boss. Just looking for a little solitude. It’s nice and quiet down here.”

“Really?” Nightmare muses, stalking closer. He finds the basement rather stimulating, but only because he can feel all the misery and despair pouring out of the victims trapped inside the walls. He supposes to anyone else it would seem peaceful, though of course that’s not why Killer’s here. “Because I’m pretty sure you were looking for companionship.”

It’s a chance for Killer to confess, not that he’ll take it. His lying smile comes easily to his face, and he shrugs so carelessly it would be easy to miss the shift of his fingers clenching in his pocket, curling around the knife Nightmare knows he keeps there. 

“Heh,” he breathes easily, rocking back on his heels. “Guess you know me better than I know myself, Boss.” 

Deliberately, the angle of his arm shifts. He’s let go of the knife. The sideways glance he casts at Nightmare is knowing, and it’s only then the guardian realises the writhing of his tentacles has calmed in response to the withdrawn threat. Smirking broadly, Killer stalks towards him, full of renewed confidence. He leans into Nightmare’s personal space, impertinent and intimate, his soul illuminating the fanged angles of his grin.

“Did you come down here to keep me company, then?”

Nightmare’s known Killer too long to be perturbed by his brashness. He scoffs. “I came down to make use of you, since you don’t seem to appreciate the free time I grant you.”

Killer’s soul is flickering, still not quite back to its proper rounded shape. He’s learned to mask his feelings from Nightmare, but there’s no stopping the streaks of black dripping furiously out of his eye-sockets as his emotions roil. For all his suave ease, Killer knows this conversation could easily turn dangerous. Nightmare allows him the illusion of keeping secrets simply because it amuses him that Killer would dare, but he could easily flay Killer open with the truth of all the things he knows.

Of all the times Killer has finished a mission early just so he can spend a few furtive minutes with Color, pretending at friendship.

Of the cats he keeps smuggling into his room, feeding and fattening them up before dropping them back in more peaceful AUs for someone else to adopt. 

Of the wisps of doubt and discomfort that stir in him every so often when he’s left on his own, like he’s starting to rethink his loyalties.

A fat drip of liquid hate rolls down from Killer’s socket, catching on the edge of his jawline and dangling precariously for a moment before splattering down against his neck. Almost thoughtlessly, Nightmare reaches out and smears the mark, tracing a line like a cut across Killer’s throat. Killer is as still as a statue, neither yielding nor flinching. Nightmare unleashes a smirk of his own, sharp enough to cut as he makes his demand.

“Get down on your knees.”

After a tense moment of pause -- long enough for both of them to consider the hand Killer still has in his pocket, close to the knife -- Killer obeys, sinking down with an unhurried grace. Carefully, he offers his hands, displaying them palm up and open as though to quell the temper Nightmare isn’t allowing himself to show.

He’s less appeased by the lack of aggression than the sight of Killer bowed before him, not wholly subjugated but willing enough. With a soft purr of satisfaction, Nightmare reaches for him.

“Tell me, Killer,” Nightmare begins softly. His tentacles come up to cradle the back of Killer’s skull, both to brace him and hold him still as Nightmare hooks a finger into his eye-socket. Killer’s magic swarms against the intrusion, trying to push the obstruction back out, albeit without success. It feels his phalange is immersed in a hot liquid - not quite enough to scald, but prickling with a warning heat. “Do you like it here?”

“Is that a trick question?” Killer asks, managing to sound curious and amused rather than wary. He’s very carely not to blink or balk as Nightmare carefully probes the inside of his socket, gently stroking the interior. One wrong pull or scratch would leave him wounded in a very delicate and painful place, but so long as he doesn’t resist Nightmare will keep his touch gentle. 

“Not at all,” Nightmare says genially, enjoying the subtle shifts of emotions as Killer struggles to keep his expression calm. “This is your home, after all. We both know there’s nothing else for you, no other place you can go. It would upset me to think you don’t appreciate your place here.”

The inside of Killer’s sockets are already slick from their usual secretions. Nightmare’s finger slides easily, tracing playful patterns on the inside of his lower lid. Killer’s breath catches, teeth parting as he breathes more heavily in response. His sockets go even wider when Nightmare pulls out his cock, its teal glow a poisonous contrast to Killer’s warm red. 

“You would never be ungrateful,” Nightmare continues, his tentacles soothingly stroking Killer’s skull as he tilts his subordinate’s face upward to a more accessible angle. “Especially not when I’ve come to treat you with such favour.”

He directs a tentacle to swipe some of the excess liquid from Killer’s face, before curling the wet tendril back around his shaft. He strokes himself indulgently, slicking himself with Killer’s fluids, the dark secretions oozing between his finger-bones and marring the bright glow of his ectoflesh. 

“Y-yes,” Killer agrees, seemingly mesmerised by the sight. Nightmare can feel the desire rising in him, adding a slight sting to the taste of his feelings like a meal laced with too much chilli -- a spicy bite to an otherwise familiar meal.

“You want this?” Nightmare teases. Killer’s already leaning forward, mouth wet and willing, but with a slight twist Nightmare adjusts the angle of his face until the head of his cock is resting on the lip of Killer’s eye-socket. Killer tenses but doesn’t struggle, radiating both anticipation and dread.

“Come on, Boss,” Killer pleads. His soul shudders, and there’s a tangled snarl in his feelings that, for a moment, pulses with unease and resentment and refusal. It’s absolutely delicious, especially the stronger burst of shock when Nightmare shoves him forward, his cock sinking deeply into Killler’s socket. Killer’s body seizes, taut with the intensity of sensation that’s too sharp to be only pleasure but too satisfying to be merely pain. His expression loses its insufferable grin, replaced by mindless desperation. “Oh fuck, oh fuck-”

“You want this,” Nightmare repeats, not a question this time, but a command. 

“I want this,” Killer echos hazily, his expression contorted and unfocused. Hard to blame him when the head of Nightmare’s cock is rubbing up against the inside of his skull, contaminating his magic and drowning out his thoughts. Their magic is fully entwined, but Nightmare has all the control and all the power.

“This is your home,” Nightmare says, each word like a brand imprinting on Killer’s soul in this suggestible state. “This is where you belong.”

“This is where...I belong,” Killer says sluggishly. There’s a vibrant glow at the front of his shorts, but his hands stay limp at his sides, passive and pliant. As a reward, Nightmare lets a tentacle curl over the bulge, stroking and squeezing gently. Killer makes a beautiful whimper of sound, breath stuttering.

“You’re mine,” Nightmare insists more aggressively. Killer’s not allowed to leave. He’s not allowed to entertain thoughts of others, of freedom. Nightmare’s claim is more than bone-deep. His magic has seeped into Killer’s marrow, into his very soul. Even if he could overcome his LV, there’s nothing he could do to revoke Nightmare’s claim. “Mine!”

“Yours,” Killer agrees shakily. The heat and pressure of the magic inside his skull squeezes tightly around Nightmare’s cock, and it takes only a few sharp, short thrusts to push him to orgasm. Killer gurgles wetly, hands spasming as Nightmare’s essence flows into him. A few simple tweaks to the structure of his magic unleashes a deluge rather than the pitiful trickle of a typical ejaculation. He wants to completely saturate and overwhelm Killer’s magic, and is inordinately gratified when the excess wells up and starts to flow out of Killer’s other socket as well as from the corners of his mouth. 

The shade of Nightmare’s magic is tinted faintly blue rather than the rusty brown-black of liquid hate. Unless someone looks closely, the two are practically indistinguishable, and Nightmare relishes the thought of Killer openly wearing Nightmare’s come on his face with no one else the wiser. 

“Well done,” Nightmare murmurs, pulling his cock free of Killer’ skull. It’s dripping wet and filthy with both his and Killer’s fluids, but he pulls his shorts back up with only faintly acknowledged discomfort. “But you never answered my question.”

“Hm?” Killer is still dazed and slow to react. He tilts his head, confused. 

“Do you like it here, Killer?” Nightmare asks benevolent, as if he’d allow Killer to truly speak his mind.

“Of course, Boss,” Killer agrees, immediate and fervent. Though his expression is slow to form, it settles back into its usual inscrutable smirk. His soul is back to its proper shape, a perfect circle surrounded by a ring of red. “I belong here. Besides, ain’t I yours?”

It’s exactly what Nightmare wants to hear. Approvingly, he squeezes his tentacle around the almost forgotten bulge of Killer’s own cock still jutting between his legs. Killer makes a small, startled sound, body arching readily into the calculated touch. 

“That’s right,” Nightmare purrs, pushing Killer back against the floor. “And I take good care of things that are mine. Let me show you.”


End file.
